Silicon nitride which is a ceramic promising as a high temperature structural material is hard to sinter, and, heretofore, it is general to prepare a sintered body therefrom using an oxide such as Y.sub.2 O.sub.3 or Al.sub.2 O.sub.3 as a sintering aid(s). By using these sintering aids, there have become possible dense sintering at normal pressure and application to parts of complicated shapes. Further, as for .alpha.-sialon sintered bodies, the present inventors disclosed in Japanese Laid-Open Patent Publication Nos. 260471/1985, 260472/1985 and 91065/1986 that sintered bodies having excellent mechanical characteristics could be obtained by a normal pressure sintering method.
However, the oxide assistants are usually contained in an amount of the order of 10 wt % in total in such a silicon nitride sintered body, and often remain as an intergranular phase in the sintered body. In .alpha.-sialon sintered bodies, the same phenomenon can take place in that part of the dissolved elements remains in the grain boundary. When silicon nitride ceramics are used as high temperature structural components, the presence of the intergranular phase in the sintered bodies is one of causes whereby lowering of high temperature strength and corrosion resistance is brought about, and for solution thereof, investigations of additives and researches on crystallization treatment of the intergranular phase, etc. are extensively made.
For example, W. Braue et al. reported in Proc. of International Symposium on Ceramics Components for Engine, 1986 FRG, items 503 to 510 that high temperature strength characteristics are enhanced by intergranular crystallization of silicon nitride using Y.sub.2 O.sub.3 or Al.sub.2 O.sub.3 as a sintering assistant.
.alpha.-sialon is a substance having structure such that Al and O took the place of and dissolved at the Si and N positions of the .alpha.-Si.sub.3 N.sub.4 crystalline structure, respectively, and at the same time, metal elements such as Li, Mg and Y made an invasion between the crystal lattices and dissolved therein, and has a characteristic that it is stable at high temperatures. Metal elements which make an invasion and dissolve are added usually in the form of an oxide, part of these oxides remain as intergranular phases without dissolving. Although several % of SiO.sub.2 is contained as an impurity due to surface oxidation, etc. in silicon nitride as a starting material, it is known that such SiO.sub.2 often reacts with Y.sub.2 O.sub.3 or the like as the dissolved element in .alpha.-sialon and the product remains as intergranular phases, and the presence of these intergranular phases becomes a cause whereby lowering of high temperature characteristics and corrosion resistance is brought about.
Although researches for reduction of the amount of the intergranular phases and enhancement of characteristics by crystallization treatment, etc. have so far been made, the present inventors found as a result of intense researches that although crystallization treatment, etc. are effective for improvement of high temperature strength characteristics, long time oxidation at a temperature of 1200.degree. C. or higher causes problems particularly on corrosion resistance, for example that foams are formed and/or cracks are formed on the surface.
This invention aims to provide a highly corrosion resistant .alpha.-sialon-like sintered body, for solving the above problems, capable of withstanding use under an environment where not only mechanical characteristics such as strength and toughness, but corrosion resistance such as oxidation resistance or chemical resistance are required, and a process for preparation thereof.
The present inventors had made intense researches for solving the above problems, and as a result they found, now, that a sintered body obtained by mixing Sc.sub.2 O.sub.3 incapable of dissolving in .alpha.-sialon with an .alpha.-sialon composition consisting of silicon nitride, aluminum nitride and an oxide of metal element M wherein M represents Yb, Er or Dy, molding the resultant mixture, and then sintering the molding in a non-oxidizing atmosphere at a temperature within the range of 1600.degree. to 2000.degree. C., and a sintered body obtained by further heat treating the above sintered body according to necessity are excellent not only in mechanical characteristics, but in corrosion resistance.